Been there, done that: The problem with quantum research

Progress in quantum physics appears only in the title of the talk.

The Dutch quantum optics community is quite a strong one, so it has a session devoted to its work at Physics@FOM almost every year. However, over the years, I've started to become a bit cynical about the work—and the work done by much of the rest of the world on the same topic.

When the quantum optics field began, a lot of experimental "gotchas" were swept under the carpet. The results we obtained were not in doubt, but the efficiency was low—days of measurement to get a single data point. Now, as things move forward, researchers are starting to talk about applications—one of the talks was partially titled "Towards the quantum Internet"—meaning that these inefficiencies need to be dealt with. Yet the approach by people in the field seems to be to repeat what has been done before and hope that it all gets better somehow.

Starving the quantum Internet of bandwidth

Indeed, in a talk that promised quantum Internet, there was no Internet involved. Very little data transfer was talked about, and critical points went unaddressed. In the work, researchers entangled two nitrogen vacancies via their photon emissions. This entanglement was used to teleport a quantum state from one vacancy to another, which may sound radical, but it has been demonstrated in a variety of systems now.

Somehow, this is supposed to be the beginning of an Internet, I guess.

Let's take a step back. When set in a diamond crystal, a nitrogen atom leaves an electron hanging around, unable to bond. This electron behaves very much like an electron attached to a single ion. It has very well-defined states, and these states can be set by shining laser light and microwaves on the atom. That is what the researchers have done: shine the light on two of these and set their states to a defined ground state.

By giving a nitrogen vacancy a short blast of microwave radiation, it is put into a superposition of two states: the ground state and a low-lying state right next to it. After that, it's hit with more laser light. If the nitrogen vacancy is in one state, it will emit a photon; otherwise it won't. Mix the light from the two nitrogen vacancies at a beam splitter, and because you don't know which vacancy emitted a photon, they become entangled.

That's the most condensed version of an entanglement experiment that you will read. Essentially, the light is used to tell us which state we're in, while the microwaves are used to set the state. By choosing just the right amount of microwave energy, there is exactly a 50/50 chance that the vacancy will end up in state one and an equal chance it will end up in state two. Because we don't look, it behaves like it's in both. When we shine light on the vacancy, we're asking "are you in state one?" If it emits light, the answer is yes, if it doesn't, the answer is no.

In most cases, we end up with two nitrogen vacancy centers and only one photon. If we carefully set up the apparatus so that we don't know center emitted the photon, we have to describe the system as a single emitter: that is, the two nitrogen centers are entangled. If this sounds a bit like a hustler's game, that's because it is. The objects are entangled only because we choose not to look.

And then there's the efficiency. For every million light pulses, the two centers are only successfully entangled once because most of the laser photons that we send their way are ignored. And this is going to become the backbone of a quantum Internet? As for a quantum Internet, if it has an error rate of 106, you can keep it.

And then there's the fact that quantum teleportation has been demonstrated multiple times by now. I'm a great fan of quantum physics and entanglement experiments, but I would love to see experiments that went beyond what we have seen already. The experiments presented here were new only in the materials used—not in the techniques or the physical outcome.

Even worse, the one interesting question from the talk was not answered. To entangle the two nitrogen vacancies, the light they emit is mixed using a partially reflective mirror. The key is that the experimenter must not be able to tell which emitter the light came from. But if the light reflects off, the mirror will get a momentum kick. In other words, we might in principle be able to determine where the photon came from without disturbing the photon's path through the system. Is the entanglement destroyed by this? Admittedly, this is a very deep question, but the speaker did not even attempt to address it.

Smoke and quantum mirrors

The next talk, on quantum states of mirrors and light, was along the same lines. Normally, this is a subject that I find... troubling. The physicists who do this work talk about how incredible it is that a macroscopic object should enter into a superposition of multiple states. A superposition is just a mixture of two (or more) states, and it is trivial to get a macroscopic superposition. For instance, if I pluck a string on a guitar in the right way, I will get several harmonics. That is a superposition of vibrational frequencies on a macroscopic string. How is that any different from a superposition of vibrational modes of a mirror?

So where exactly does the amazement come from? The only difference is that these physicists want to couple a decidedly quantum object—a photon—with a decidedly non-quantum object. Except that we're talking about coupling vibrational modes (electromagnetic and sound) in things like mirrors. That doesn't make the mirror a quantum object; it simply points to how collective behavior (vibrational modes) have similarities to certain aspects of quantum mechanics.

The technical part of the talk was interesting. After all, it's difficult to cool a mirror to the point where it is dominated by its fundamental vibrational mode. And there is no doubt that the progress is quite impressive. But the justification—references to the living ghost of Roger Penrose, quantum gravity, and other unlikely things—was incredibly weak. Cooling a mirror to its fundamental mode is interesting in and of itself.

This talk led my partner in crime to reach the conclusion that you see in the image at the beginning of this article. For those who can't read the writing, "In Delta t < 5 years (i.e. before 2019) this shit will be sold as gravity wave detector."

Chris Lee is currently attending the Physics@FOM meeting; this is one of a series of reports from that meeting.

Chris Lee / Chris writes for Ars Technica's science section. A physicist by day and science writer by night, he specializes in quantum physics and optics. He lives and works in Eindhoven, the Netherlands.